Surface energy of magnetized superconducting matter in neutron star cores

Abstract

In the outer core of a neutron star (NS) quantum liquids are thought to exist during most of the observable stellar lifetime. These liquids are characterized by two different, density-dependent superfluid (SF) condensation critical temperatures associated with neutrons and protons. A possibility that cannot be excluded in the state-of-the-art astrophysics, and that warrants examination, is a spatial overlap of the SF and superconducting (SC) domains. Partial overlap would imply that there is a layer where both critical temperatures are equal. In vicinity of this layer the Ginzburg-Landau (GL) effective field theory is valid. In this paper, an effective field theory for a proton superconductor (SC) interacting with a neutron SF, both with scalar order parameters, is developed and applied to the surface energy (SE) of a magnetized SC body. Essentially, the SE studied here differs from the nuclear SE: here, the proton SF density decays to zero while the total proton density is constant across the surface. Interactions between the condensates are parametrized phenomenologically and their effects determined from calculations of a planar SE as the ranges of parameters are varied. The critical GL parameter κc which renders the SE equal to zero is found analytically by noting that in a system with vanishing SE the thermodynamic critical MF (magnetic field) is equivalent to the upper critical MF. In the case of weak coupling, κc is shown to be a linear function of SF-SF density-density coupling, in agreement with the earlier results based on asymptotic intervortex interactions. Numerical simulations corroborate my analytical predictions. Coupling due to the mixed term arising from a scalar product of gradients of the SF densities, which had been considered in the earlier literature, is seen to have practically no effect on the superconductivity type. However, this coupling does produce a frozen wave packet of the SF neutron density localized at the surface. It is shown that the leading contribution from the gradient coupling arises from a novel mixed quantum pressure term, but still does not affect the planar SE. The present calculations provide an initial map of superconductivity types in the phenomenological effective field theory and will serve as a landmark for future studies, which require microscopic calculations of the coupling parameters introduced here phenomenologically.

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